1. Field of the Invention
The present invention relates to an improved converging type solar cell element, especially to one in which electrodes are formed on the back of a silicon substrate.
2. Description of the Related Art
Solar cells are used for various purposes. For the purpose of achieving a reduction in costs of a power generation system utilizing solar cells, various converging type solar cell modules that decrease an application area of expensive solar cells by converging sunlight using a lens have been originated. Also, various sun tracking systems have been conceived for the purposes of improving efficiency of power generation of converging type solar cell modules.
Such converging type solar cell modules use a converging type solar cell element in which solar cells are formed and electrodes for transferring current are provided. When a converging spot formed by convergence of sunlight is irradiated to a sunlight receiving section of the converging type solar cell element, free electrons and positive holes are generated as carriers in the silicon substrate. These carriers are separated by p-n junction, and the free electrons are then taken out as current from electrodes via an n-layer, with positive holes current supplied via a p-layer.
The converging type solar cell element mentioned above is thin, with a thickness normally within the range of 50 to 150 .mu.m. Due to this thickness, the element is occasionally damaged during mounting work or the like. Therefore, in order to improve the yield, enhanced strength is desired. Under the circumstances, as shown in FIG. 5, it is necessary to provide an embankment section 14 around a sunlight receiving section 10 of the converging type solar cell element for the purposes of reinforcement.
However, the conventional converging type solar cell element mentioned above has such a problem that some of the generated current flows into the embankment section 14 thereby decreasing the quantity of current which can be collected, and thereby lowering photoelectric conversion efficiency. The aforementioned problem can be controlled by reducing a width d of the embankment section 14 shown in FIG. 5. However, when the width of the embankment section 14 is set to 1 mm, a carrier loss of approximately 8 to 10 percent arises, whereas, even when the width is set to 0.3 mm, the loss is 3 to 5 percent.
Also, a bus electrode for collecting current which is taken out from the electrodes is provided on the back side of the embankment section 14 shown in FIG. 5. If the width d of the embankment section 14 is reduced, the area of the bus electrode will become smaller. This causes such a problem that internal resistance of the converging type solar cell element increases.
Further, the surface of the silicon substrate has many lattice defects, and therefore carriers generated by recombination of carriers disappear. This causes such a problem that quantity of current which can be collected is decreased. This is a serious problem specially to minority carriers in a silicon substrate.